Consistent reads in a distributed transaction protocol

ABSTRACT

Methods and systems for consistent reads in a distributed transaction protocol are disclosed. A method includes: receiving, by a computing device, a request to write a revision of a data object in a dispersed storage network (DSN); sending, by the computing device, a proposal with the revision of the data object to a plurality of storage units; receiving, by the computing device, a response to the proposal from each of the plurality of storage units, the response including a proposed minimum timestamp corresponding to the data object; determining, by the computing device, a minimum timestamp for the data object based on the proposed minimum timestamps received from the plurality of storage units; and determining, by the computing device, a version of the data object written in the DSN based on the minimum timestamp.

BACKGROUND

Aspects of the present invention generally relate to computing devicesand, more particularly, to methods and systems for consistent reads in adistributed transaction protocol.

Computing devices communicate data, process data, and/or store data.Such computing devices range from wireless smart phones, laptops,tablets, personal computers (PCs), work stations, and video gamedevices, to data centers that support millions of web searches, stocktrades, or online purchases every day. A computer may effectively extendits central processing unit (CPU) by using cloud computing to performone or more computing functions (e.g., a service, an application, analgorithm, an arithmetic logic function, etc.) on behalf of thecomputer. Cloud computing is a model of service delivery for enablingconvenient, on-demand network access to a shared pool of configurablecomputing resources (e.g., networks, network bandwidth, servers,processing, memory, storage, applications, virtual machines, andservices) that can be rapidly provisioned and released with minimalmanagement effort or interaction with a provider of the service.

Further, for large services, applications, and/or functions, cloudcomputing may be performed by multiple cloud computing resources in adistributed manner to improve the response time for completion of theservice, application, and/or function. For example, Hadoop® (aregistered trademark of The Apache Software Foundation) is an opensource software framework that supports distributed applicationsenabling application execution by thousands of computers. Networkstorage is a computing capability that is typically offered by cloudcomputing providers. In particular, a user of cloud computing servicesmay store and retrieve data on cloud infrastructure maintained by acloud computing provider, such as a dispersed storage network (DSN)system. A computer may use cloud storage as part of its memory system.Cloud storage enables a user, via a computer, to store files,applications, etc., on an Internet storage system. The Internet storagesystem may include a redundant array of independent disks (RAID) systemand/or a dispersed storage system (dispersed storage network memory)that uses an error correction scheme to encode data for storage.

SUMMARY

In a first aspect of the invention, there is a method that includes:receiving, by a computing device, a request to write a revision of adata object in a dispersed storage network (DSN); sending, by thecomputing device, a proposal with the revision of the data object to aplurality of storage units; receiving, by the computing device, aresponse to the proposal from each of the plurality of storage units,the response including a proposed minimum timestamp corresponding to thedata object; determining, by the computing device, a minimum timestampfor the data object based on the proposed minimum timestamps receivedfrom the plurality of storage units; and determining, by the computingdevice, a version of the data object written in the DSN based on theminimum timestamp.

In another aspect of the invention, there is a computer program productthat includes one or more computer readable storage media, and programinstructions collectively stored on the one or more computer readablestorage media. The program instructions include: program instructions toreceive a read request for a data object, the read request including arequest timestamp; program instructions to determine a currenttimestamp; program instructions to retrieve data from storagecorresponding to the data object at the request timestamp; programinstructions to update a read timestamp cache based on the currenttimestamp; and program instructions to provide the retrieved data to adispersed storage (DS) processing unit.

In another aspect of the invention, there is a system that includes ahardware processor, a computer readable memory, and one or more computerreadable storage media associated with a computing device, wherein thecomputing device is a dispersed storage (DS) processing unit; programinstructions to receive a request to write a revision of a data objectin a dispersed storage network (DSN); program instructions to send aproposal with the revision of the data object to a plurality of storageunits; program instructions to receive a response to the proposal fromeach of the plurality of storage units, the response including aproposed minimum timestamp corresponding to the data object; programinstructions to determine a minimum timestamp for the data object basedon the proposed minimum timestamps received from the plurality ofstorage units; and program instructions to determine a version of thedata object written in the DSN based on the minimum timestamp, whereinthe program instructions are collectively stored on the one or morecomputer readable storage media for execution by the hardware processorvia the computer readable memory.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present invention are described in the detaileddescription which follows, in reference to the noted plurality ofdrawings by way of non-limiting examples of exemplary embodiments of thepresent invention.

FIG. 1 depicts a cloud computing node according to an embodiment of thepresent invention.

FIG. 2 depicts a cloud computing environment according to an embodimentof the present invention.

FIG. 3 depicts abstraction model layers according to an embodiment ofthe present invention.

FIG. 4 depicts an illustrative environment in accordance with aspects ofthe invention.

FIGS. 5 and 6 depict flowcharts of exemplary methods performed inaccordance with aspects of the invention.

DETAILED DESCRIPTION

Aspects of the present invention generally relate to computing devicesand, more particularly, to methods and systems for consistent reads in adistributed transaction protocol. As described herein, aspects of theinvention include a method and system that enable a distributedtransaction protocol for reading multiple sources (objects) consistentlyin a DSN by issuing, from a dispersed storage (DS) processing unit,multiple read requests with a particular timestamp, and reading, by a DSstorage unit, a highest revision (version) with a timestamp less than orequal to the particular timestamp. Aspects of the invention also includea method and system that enable a distributed transaction protocol that,after a client has restored a particular revision of a source, preventswriting in the DSN of any revision of that source that is lower than orequal to the particular revision that was restored.

Embodiments address problems with consistency in a DSN. In particular,embodiments address problems with particular revisions of sources beingchanged after having been read/restored by a client. Accordingly,embodiments improve the functioning of a computer by providing methodsand systems for immediately consistent reads in a distributedtransaction protocol. In particular, embodiments improve the functioningof a computer by providing a method and system that enable a distributedtransaction protocol for reading multiple sources consistently in a DSNby issuing, from a DS processing unit, multiple read requests with aparticular timestamp, and reading, by a DS storage unit, a highestrevision with a timestamp less than or equal to the particulartimestamp. Embodiments also improve the functioning of a computer byproviding a method and system that enable a distributed transactionprotocol that, after a client has restored a particular revision of asource, preserves consistency by preventing writing in the DSN of anyrevision of that source that is lower than or equal to the particularrevision that was restored. Additionally, implementations of theinvention use techniques that are, by definition, rooted in computertechnology (e.g., DSNs, DS processing units, DS storage units, and cloudcomputing).

The present invention may be a system, a method, and/or a computerprogram product at any possible technical detail level of integration.The computer program product may include a computer readable storagemedium (or media) having computer readable program instructions thereonfor causing a processor to carry out aspects of the present invention.

The computer readable storage medium can be a tangible device that canretain and store instructions for use by an instruction executiondevice. The computer readable storage medium may be, for example, but isnot limited to, an electronic storage device, a magnetic storage device,an optical storage device, an electromagnetic storage device, asemiconductor storage device, or any suitable combination of theforegoing. A non-exhaustive list of more specific examples of thecomputer readable storage medium includes the following: a portablecomputer diskette, a hard disk, a random access memory (RAM), aread-only memory (ROM), an erasable programmable read-only memory (EPROMor Flash memory), a static random access memory (SRAM), a portablecompact disc read-only memory (CD-ROM), a digital versatile disk (DVD),a memory stick, a floppy disk, a mechanically encoded device such aspunch-cards or raised structures in a groove having instructionsrecorded thereon, and any suitable combination of the foregoing. Acomputer readable storage medium or media, as used herein, is not to beconstrued as being transitory signals per se, such as radio waves orother freely propagating electromagnetic waves, electromagnetic wavespropagating through a waveguide or other transmission media (e.g., lightpulses passing through a fiber-optic cable), or electrical signalstransmitted through a wire.

Computer readable program instructions described herein can bedownloaded to respective computing/processing devices from a computerreadable storage medium or to an external computer or external storagedevice via a network, for example, the Internet, a local area network, awide area network and/or a wireless network. The network may comprisecopper transmission cables, optical transmission fibers, wirelesstransmission, routers, firewalls, switches, gateway computers and/oredge servers. A network adapter card or network interface in eachcomputing/processing device receives computer readable programinstructions from the network and forwards the computer readable programinstructions for storage in a computer readable storage medium withinthe respective computing/processing device.

Computer readable program instructions for carrying out operations ofthe present invention may be assembler instructions,instruction-set-architecture (ISA) instructions, machine instructions,machine dependent instructions, microcode, firmware instructions,state-setting data, configuration data for integrated circuitry, oreither source code or object code written in any combination of one ormore programming languages, including an object oriented programminglanguage such as Smalltalk, C++, or the like, and procedural programminglanguages, such as the “C” programming language or similar programminglanguages. The computer readable program instructions may executeentirely on the user's computer, partly on the user's computer, as astand-alone software package, partly on the user's computer and partlyon a remote computer or entirely on the remote computer or server. Inthe latter scenario, the remote computer may be connected to the user'scomputer through any type of network, including a local area network(LAN) or a wide area network (WAN), or the connection may be made to anexternal computer (for example, through the Internet using an InternetService Provider). In some embodiments, electronic circuitry including,for example, programmable logic circuitry, field-programmable gatearrays (FPGA), or programmable logic arrays (PLA) may execute thecomputer readable program instructions by utilizing state information ofthe computer readable program instructions to personalize the electroniccircuitry, in order to perform aspects of the present invention.

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer readable program instructions.

These computer readable program instructions may be provided to aprocessor of a computer, or other programmable data processing apparatusto produce a machine, such that the instructions, which execute via theprocessor of the computer or other programmable data processingapparatus, create means for implementing the functions/acts specified inthe flowchart and/or block diagram block or blocks. These computerreadable program instructions may also be stored in a computer readablestorage medium that can direct a computer, a programmable dataprocessing apparatus, and/or other devices to function in a particularmanner, such that the computer readable storage medium havinginstructions stored therein comprises an article of manufactureincluding instructions which implement aspects of the function/actspecified in the flowchart and/or block diagram block or blocks.

The computer readable program instructions may also be loaded onto acomputer, other programmable data processing apparatus, or other deviceto cause a series of operational steps to be performed on the computer,other programmable apparatus or other device to produce a computerimplemented process, such that the instructions which execute on thecomputer, other programmable apparatus, or other device implement thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the blocks may occur out of theorder noted in the Figures. For example, two blocks shown in successionmay, in fact, be accomplished as one step, executed concurrently,substantially concurrently, in a partially or wholly temporallyoverlapping manner, or the blocks may sometimes be executed in thereverse order, depending upon the functionality involved. It will alsobe noted that each block of the block diagrams and/or flowchartillustration, and combinations of blocks in the block diagrams and/orflowchart illustration, can be implemented by special purposehardware-based systems that perform the specified functions or acts orcarry out combinations of special purpose hardware and computerinstructions.

It is understood in advance that although this disclosure includes adetailed description on cloud computing, implementation of the teachingsrecited herein are not limited to a cloud computing environment. Rather,embodiments of the present invention are capable of being implemented inconjunction with any other type of computing environment now known orlater developed.

Cloud computing is a model of service delivery for enabling convenient,on-demand network access to a shared pool of configurable computingresources (e.g., networks, network bandwidth, servers, processing,memory, storage, applications, virtual machines, and services) that canbe rapidly provisioned and released with minimal management effort orinteraction with a provider of the service. This cloud model may includeat least five characteristics, at least three service models, and atleast four deployment models.

Characteristics are as follows:

On-demand self-service: a cloud consumer can unilaterally provisioncomputing capabilities, such as server time and network storage, asneeded automatically without requiring human interaction with theservice's provider.

Broad network access: capabilities are available over a network andaccessed through standard mechanisms that promote use by heterogeneousthin or thick client platforms (e.g., mobile phones, laptops, and PDAs).

Resource pooling: the provider's computing resources are pooled to servemultiple consumers using a multi-tenant model, with different physicaland virtual resources dynamically assigned and reassigned according todemand. There is a sense of location independence in that the consumergenerally has no control or knowledge over the exact location of theprovided resources but may be able to specify location at a higher levelof abstraction (e.g., country, state, or datacenter).

Rapid elasticity: capabilities can be rapidly and elasticallyprovisioned, in some cases automatically, to quickly scale out andrapidly released to quickly scale in. To the consumer, the capabilitiesavailable for provisioning often appear to be unlimited and can bepurchased in any quantity at any time.

Measured service: cloud systems automatically control and optimizeresource use by leveraging a metering capability at some level ofabstraction appropriate to the type of service (e.g., storage,processing, bandwidth, and active user accounts). Resource usage can bemonitored, controlled, and reported providing transparency for both theprovider and consumer of the utilized service.

Service Models are as follows:

Software as a Service (SaaS): the capability provided to the consumer isto use the provider's applications running on a cloud infrastructure.The applications are accessible from various client devices through athin client interface such as a web browser (e.g., web-based e-mail).The consumer does not manage or control the underlying cloudinfrastructure including network, servers, operating systems, storage,or even individual application capabilities, with the possible exceptionof limited user-specific application configuration settings.

Platform as a Service (PaaS): the capability provided to the consumer isto deploy onto the cloud infrastructure consumer-created or acquiredapplications created using programming languages and tools supported bythe provider. The consumer does not manage or control the underlyingcloud infrastructure including networks, servers, operating systems, orstorage, but has control over the deployed applications and possiblyapplication hosting environment configurations.

Infrastructure as a Service (IaaS): the capability provided to theconsumer is to provision processing, storage, networks, and otherfundamental computing resources where the consumer is able to deploy andrun arbitrary software, which can include operating systems andapplications. The consumer does not manage or control the underlyingcloud infrastructure but has control over operating systems, storage,deployed applications, and possibly limited control of select networkingcomponents (e.g., host firewalls).

Deployment Models are as follows:

Private cloud: the cloud infrastructure is operated solely for anorganization. It may be managed by the organization or a third party andmay exist on-premises or off-premises.

Community cloud: the cloud infrastructure is shared by severalorganizations and supports a specific community that has shared concerns(e.g., mission, security requirements, policy, and complianceconsiderations). It may be managed by the organizations or a third partyand may exist on-premises or off-premises.

Public cloud: the cloud infrastructure is made available to the generalpublic or a large industry group and is owned by an organization sellingcloud services.

Hybrid cloud: the cloud infrastructure is a composition of two or moreclouds (private, community, or public) that remain unique entities butare bound together by standardized or proprietary technology thatenables data and application portability (e.g., cloud bursting forload-balancing between clouds).

A cloud computing environment is service oriented with a focus onstatelessness, low coupling, modularity, and semantic interoperability.At the heart of cloud computing is an infrastructure comprising anetwork of interconnected nodes.

Referring now to FIG. 1, a schematic of an example of a cloud computingnode is shown. Cloud computing node 10 is only one example of a suitablecloud computing node and is not intended to suggest any limitation as tothe scope of use or functionality of embodiments of the inventiondescribed herein. Regardless, cloud computing node 10 is capable ofbeing implemented and/or performing any of the functionality set forthhereinabove.

In cloud computing node 10 there is a computer system/server 12, whichis operational with numerous other general purpose or special purposecomputing system environments or configurations. Examples of well-knowncomputing systems, environments, and/or configurations that may besuitable for use with computer system/server 12 include, but are notlimited to, personal computer systems, server computer systems, thinclients, thick clients, hand-held or laptop devices, multiprocessorsystems, microprocessor-based systems, set top boxes, programmableconsumer electronics, network PCs, minicomputer systems, mainframecomputer systems, and distributed cloud computing environments thatinclude any of the above systems or devices, and the like.

Computer system/server 12 may be described in the general context ofcomputer system executable instructions, such as program modules, beingexecuted by a computer system. Generally, program modules may includeroutines, programs, objects, components, logic, data structures, and soon that perform particular tasks or implement particular abstract datatypes. Computer system/server 12 may be practiced in distributed cloudcomputing environments where tasks are performed by remote processingdevices that are linked through a communications network. In adistributed cloud computing environment, program modules may be locatedin both local and remote computer system storage media including memorystorage devices.

As shown in FIG. 1, computer system/server 12 in cloud computing node 10is shown in the form of a general-purpose computing device. Thecomponents of computer system/server 12 may include, but are not limitedto, one or more processors or processing units 16, a system memory 28,and a bus 18 that couples various system components including systemmemory 28 to processor 16.

Bus 18 represents one or more of any of several types of bus structures,including a memory bus or memory controller, a peripheral bus, anaccelerated graphics port, and a processor or local bus using any of avariety of bus architectures. By way of example, and not limitation,such architectures include Industry Standard Architecture (ISA) bus,Micro Channel Architecture (MCA) bus, Enhanced ISA (EISA) bus, VideoElectronics Standards Association (VESA) local bus, and PeripheralComponent Interconnects (PCI) bus.

Computer system/server 12 typically includes a variety of computersystem readable media. Such media may be any available media that isaccessible by computer system/server 12, and it includes both volatileand non-volatile media, removable and non-removable media.

System memory 28 can include computer system readable media in the formof volatile memory, such as random access memory (RAM) 30 and/or cachememory 32. Computer system/server 12 may further include otherremovable/non-removable, volatile/non-volatile computer system storagemedia. By way of example only, storage system 34 can be provided forreading from and writing to a nonremovable, non-volatile magnetic media(not shown and typically called a “hard drive”). Although not shown, amagnetic disk drive for reading from and writing to a removable,non-volatile magnetic disk (e.g., a “floppy disk”), and an optical diskdrive for reading from or writing to a removable, non-volatile opticaldisk such as a CD-ROM, DVD-ROM or other optical media can be provided.In such instances, each can be connected to bus 18 by one or more datamedia interfaces. As will be further depicted and described below,memory 28 may include at least one program product having a set (e.g.,at least one) of program modules that are configured to carry out thefunctions of embodiments of the invention.

Program/utility 40, having a set (at least one) of program modules 42,may be stored in memory 28 by way of example, and not limitation, aswell as an operating system, one or more application programs, otherprogram modules, and program data. Each of the operating system, one ormore application programs, other program modules, and program data orsome combination thereof, may include an implementation of a networkingenvironment. Program modules 42 generally carry out the functions and/ormethodologies of embodiments of the invention as described herein.

Computer system/server 12 may also communicate with one or more externaldevices 14 such as a keyboard, a pointing device, a display 24, etc.;one or more devices that enable a user to interact with computersystem/server 12; and/or any devices (e.g., network card, modem, etc.)that enable computer system/server 12 to communicate with one or moreother computing devices. Such communication can occur via Input/Output(I/O) interfaces 22. Still yet, computer system/server 12 cancommunicate with one or more networks such as a local area network(LAN), a general wide area network (WAN), and/or a public network (e.g.,the Internet) via network adapter 20. As depicted, network adapter 20communicates with the other components of computer system/server 12 viabus 18. It should be understood that although not shown, other hardwareand/or software components could be used in conjunction with computersystem/server 12. Examples, include, but are not limited to: microcode,device drivers, redundant processing units, external disk drive arrays,RAID systems, tape drives, and data archival storage systems, etc.

Referring now to FIG. 2, illustrative cloud computing environment 50 isdepicted. As shown, cloud computing environment 50 comprises one or morecloud computing nodes 10 with which local computing devices used bycloud consumers, such as, for example, personal digital assistant (PDA)or cellular telephone 54A, desktop computer 54B, laptop computer 54C,and/or automobile computer system 54N may communicate. Nodes 10 maycommunicate with one another. They may be grouped (not shown) physicallyor virtually, in one or more networks, such as Private, Community,Public, or Hybrid clouds as described hereinabove, or a combinationthereof. This allows cloud computing environment 50 to offerinfrastructure, platforms and/or software as services for which a cloudconsumer does not need to maintain resources on a local computingdevice. It is understood that the types of computing devices 54A-N shownin FIG. 2 are intended to be illustrative only and that computing nodes10 and cloud computing environment 50 can communicate with any type ofcomputerized device over any type of network and/or network addressableconnection (e.g., using a web browser).

Referring now to FIG. 3, a set of functional abstraction layers providedby cloud computing environment 50 (FIG. 2) is shown. It should beunderstood in advance that the components, layers, and functions shownin FIG. 3 are intended to be illustrative only and embodiments of theinvention are not limited thereto. As depicted, the following layers andcorresponding functions are provided:

Hardware and software layer 60 includes hardware and softwarecomponents. Examples of hardware components include: mainframes 61; RISC(Reduced Instruction Set Computer) architecture based servers 62;servers 63; blade servers 64; storage devices 65; and networks andnetworking components 66. In some embodiments, software componentsinclude network application server software 67 and database software 68.

Virtualization layer 70 provides an abstraction layer from which thefollowing examples of virtual entities may be provided: virtual servers71; virtual storage 72; virtual networks 73, including virtual privatenetworks; virtual applications and operating systems 74; and virtualclients 75.

In one example, management layer 80 may provide the functions describedbelow. Resource provisioning 81 provides dynamic procurement ofcomputing resources and other resources that are utilized to performtasks within the cloud computing environment. Metering and Pricing 82provide cost tracking as resources are utilized within the cloudcomputing environment, and billing or invoicing for consumption of theseresources. In one example, these resources may comprise applicationsoftware licenses. Security provides identity verification for cloudconsumers and tasks, as well as protection for data and other resources.User portal 83 provides access to the cloud computing environment forconsumers and system administrators. Service level management 84provides cloud computing resource allocation and management such thatrequired service levels are met. Service Level Agreement (SLA) planningand fulfillment 85 provide pre-arrangement for, and procurement of,cloud computing resources for which a future requirement is anticipatedin accordance with an SLA.

Workloads layer 90 provides examples of functionality for which thecloud computing environment may be utilized. Examples of workloads andfunctions which may be provided from this layer include: mapping andnavigation 91; software development and lifecycle management 92; virtualclassroom education delivery 93; data analytics processing 94;transaction processing 95; and DSN 96.

Referring back to FIG. 1, the program/utility 40 may include one or moreprogram modules 42 that generally carry out the functions and/ormethodologies of embodiments of the invention as described herein (e.g.,such as the functionality provided by DSN 96). Specifically, the programmodules 42 may enable a distributed transaction protocol for readingmultiple sources consistently. Additionally, the program modules 42 mayprevent writing of any revision of a source that is lower than or equalto the highest revision that has been read/restored by a client. Otherfunctionalities of the program modules 42 are described further hereinsuch that the program modules 42 are not limited to the functionsdescribed above. Moreover, it is noted that some of the program modules42 can be implemented within the infrastructure shown in FIGS. 1-3. Forexample, the program modules 42 may be representative of a DS processingunit program module 420 and a DS storage unit program module 440 asshown in FIG. 4.

FIG. 4 depicts an illustrative environment 400 in accordance withaspects of the invention. As shown, the environment 400 comprises atleast one DS processing unit 410, a plurality of DS storage units 430-1,430-2, . . . , 430-n, and a client computer system 460 which are incommunication via a computer network 470. In embodiments, the computernetwork 470 is any suitable network including any combination of a LAN,WAN, or the Internet. In embodiments, the DS processing unit 410, theplurality of DS storage units 430-1, 430-2, . . . , 430-n, and theclient computer system 460 are physically collocated, or, moretypically, are situated in separate physical locations.

The quantity of devices and/or networks in the environment 400 is notlimited to what is shown in FIG. 4. In practice, the environment 400 mayinclude additional devices and/or networks; fewer devices and/ornetworks; different devices and/or networks; or differently arrangeddevices and/or networks than illustrated in FIG. 4. Also, in someimplementations, one or more of the devices of the environment 400 mayperform one or more functions described as being performed by anotherone or more of the devices of the environment 400.

In embodiments, the DS processing unit 410 in the environment 400 issituated in the cloud computing environment 50 and is one or more of thenodes 10 shown in FIG. 2. The DS processing unit 410 is implemented ashardware and/or software using components such as mainframes 61; RISC(Reduced Instruction Set Computer) architecture based servers 62;servers 63; blade servers 64; storage devices 65; networks andnetworking components 66; virtual servers 71; virtual storage 72;virtual networks 73, including virtual private networks; virtualapplications and operating systems 74; and virtual clients 75 shown inFIG. 3.

In embodiments, the DS processing unit 410 includes a DS processing unitprogram module 420 which comprises one or more of the program modules 42shown in FIG. 1. In embodiments, the DS processing unit program module420 includes program instructions for storing data objects (sources) andretrieving data objects using the plurality of DS storage units 430-1,430-2, . . . , 430-n, as discussed herein. In embodiments, the programinstructions included in the DS processing unit program module 420 ofthe DS processing unit 410 are executed by one or more hardwareprocessors. In embodiments, the DS processing unit 410 also includes arelative system clock 425, a logical counter 426, and a relative hybridlogical clock 427, each of which may be implemented using any type ofmemory (e.g., RAM) or storage device (e.g., hard disk drive, solid statedrive, etc.) and/or one or more of the program modules 42 includingprogram instructions that are executed by one or more hardwareprocessors.

Still referring to FIG. 4, in embodiments, each of the plurality of DSstorage units 430-1, 430-2, . . . , 430-n in the environment 400 issituated in the cloud computing environment 50 and is one or more of thenodes 10 shown in FIG. 2. Each of the plurality of DS storage units430-1, 430-2, . . . , 430-n is implemented as hardware and/or softwareusing components such as mainframes 61; RISC (Reduced Instruction SetComputer) architecture based servers 62; servers 63; blade servers 64;storage devices 65; networks and networking components 66; virtualservers 71; virtual storage 72; virtual networks 73, including virtualprivate networks; virtual applications and operating systems 74; andvirtual clients 75 shown in FIG. 3.

In embodiments, each of the plurality of DS storage units 430-1, 430-2,. . . , 430-n includes a DS storage unit program module 440 and a datastorage 450. In an example, the DS storage unit program module 440comprises one or more of the program modules 42 shown in FIG. 1. Inembodiments, the DS storage unit program module 440 includes programinstructions for storing slices of data objects and retrieving slices ofdata objects in the data storage 450 in accordance with instructionsreceived from the DS processing unit program module 420 of the DSprocessing unit 410.

In embodiments, the data storage 450 receives and stores data inaccordance with instructions received from the DS storage unit programmodule 440. The data storage 450 is any type of data storage device orsystem (e.g., storage device 65 of FIG. 3) and is located on (or isaccessible to) the data storage unit 430-1, 430-2, . . . , 430-n. Inother embodiments, the data storage 450 is a storage resource providedby a cloud computing provider on another cloud computing node 10 orother location associated with the cloud computing provider that isexternal to but accessible from the data storage unit 430-1, 430-2, . .. , 430-n.

In embodiments, each of the plurality of DS storage units 430-1, 430-2,. . . , 430-n also includes a system clock 455, a logical counter 456,an absolute clock 457, and a read timestamp cache 458, each of which maybe implemented using any type of memory (e.g., RAM) or storage device(e.g., hard disk drive, solid state drive, etc.) and/or one or more ofthe program modules 42 including program instructions that are executedby one or more hardware processors.

Still referring to FIG. 4, in embodiments, the client computer system460 in the environment 400 includes one or more components of thecomputer system/server 12 (as shown in FIG. 1). In other embodiments,the client computer system 460 in the environment 400 is situated in thecloud computing environment 50 and is one or more of the nodes 10 shownin FIG. 2. In an example, the client computer system 460 is implementedas hardware and/or software using components such as mainframes 61; RISC(Reduced Instruction Set Computer) architecture based servers 62;servers 63; blade servers 64; storage devices 65; networks andnetworking components 66; virtual servers 71; virtual storage 72;virtual networks 73, including virtual private networks; virtualapplications and operating systems 74; and virtual clients 75 shown inFIG. 3. In other embodiments, the client computer system 460 is adesktop computer, a laptop computer, a mobile device such as a cellularphone, tablet, personal digital assistant (PDA), or any other computingdevice.

In embodiments, to read multiple sources consistently, the DS processingunit 410 issues multiple read requests with the same client timestampTc, as described below with reference to FIG. 5. For each read, the DSprocessing unit 410 issues requests with Tc and ensures that either thehighest revision less than or equal to Tc or the appropriate error isreturned. In embodiments, when a quorum of DS storage units 430-1,430-2, . . . , 430-n cannot service a read at a particular timestamp Tcbecause old data has already been discarded, the DS processing unit 410determines that the read is prehistoric. In embodiments, in the casethat any read fails with a prehistoric read error, the DS processingunit 410 aborts and retries the entire transaction. In embodiments, forpoint-in-time reads where a user (e.g., of the client computer system460) specifies Tc explicitly, any prehistoric read errors are returnedto the user.

In embodiments, once a client such as the client computer system 460 hasrestored (read) a revision R of a source stored in the data storageunits 430-1, 430-2, . . . , 430-n, the DS processing unit 410 prohibitsthe writing of any revision less than or equal to R of that source tothe data storage units 430-1, 430-2, . . . , 430-n. In other words, theDS processing unit 410 prohibits the writing of a version of a sourcewith a timestamp that is the same as or earlier than the timestamp of aversion of the source that has been restored by a client such as theclient computer system 460. In this manner, the DS processing unit 410guarantees that two reads of a source at any timestamp T by clients suchas the client computer system 460 always produce the same result, andthe history of the source never changes.

In embodiments, each of the at least one DS processing unit 410maintains the relative hybrid logical clock 427, which is used togenerate new timestamp revisions for writes. Each of the DS storageunits 430-1, 430-2, . . . , 430-n maintains a read timestamp cache 458which includes the most recent read timestamp for each source stored inthe data storage 450. In embodiments, in order to prevent the readtimestamp cache 458 from growing too large, the DS storage units 430-1,430-2, . . . , 430-n estimate the last read time of a given SliceName ina conservative manner. In other words, the DS storage units 430-1,430-2, . . . , 430-n may return a more recent timestamp than the truetimestamp but not an older timestamp. In embodiments, if the DS storageunits 430-1, 430-2, . . . , 430-n return a more recent timestamp thanthe true timestamp, unnecessary write retries may result. Accordingly,more space may be allocated to the read timestamp cache 458 ifperformance considerations are more important than space considerations,and less space may be allocated to the read timestamp cache 458 if spaceconsiderations are more important than performance considerations.

In embodiments, the DS storage units 430-1, 430-2, . . . , 430-ndistinguish a last retained revision (i.e., earliest retained revision)for each source, including a prehistoric timestamp revision indicating apoint where older promoted revisions have been removed in read/proposeresponse. In embodiments, DS storage units 430-1, 430-2, . . . , 430-nthat do not maintain history may safely add the prehistoric revision toany source where the predecessor of the oldest known revision is greaterthan not-found.

In embodiments, each of the at least one DS processing unit 410 and theDS storage units 430-1, 430-2, . . . , 430-n participate in conteststhat include two decisions regarding sources: MIN-TIMESTAMP (minimumtimestamp), which is a consensus on a smallest (earliest) acceptablerevision of a source that can win the contest (single-source, replicateddecision); and WINNING-SUCCESSOR, which is a decision on the exactrevision of a source to accept, limited by the result of MIN-TIMESTAMP.In embodiments, each decision has its own votes with its own set ofrounds. In particular, each of the at least one DS processing unit 410makes proposals for both the MIN-TIMESTAMP and WINNING-SUCCESSORdecisions. In embodiments, on each of the DS storage units 430-1, 430-2,. . . , 430-n, the read timestamp cache 458 is updated whenever a clientsuch as client computer system 460 issues a timestamp read. Inembodiments, the read timestamp cache 458 allows most reads to proceedwithout opening a contest or making any proposals. To that end, inembodiments, when a new contest is created on one of the DS storageunits 430-1, 430-2, . . . , 430-n, the DS storage unit 430-1, 430-2, . .. , 430-n automatically votes for the current value of its readtimestamp cache 458 in round 0 of the MIN-TIMESTAMP decision unless thefirst proposal is a MIN-TIMESTAMP proposal for a higher timestamp or ina round greater than 0.

In embodiments, since the WINNING-SUCCESSOR decision depends on thechosen MIN-TIMESTAMP, the at least one DS processing unit 410 resolvesthe MIN-TIMESTAMP vote before deciding the WINNING-SUCCESSOR. Inembodiments, the minimum timestamp is chosen on a per-source basis(i.e., per DS storage unit 430-1, 430-2, . . . , 430-n), while thewinning successor must win on all of its sources (i.e., all of thestorage units 430-1, 430-2, . . . , 430-n). In embodiments, votes by theDS storage units 430-1, 430-2, . . . , 430-n based on timestamps storedin the read timestamp cache 458 will typically allow the MIN-TIMESTAMPdecision to be made immediately, so a writer can still succeed in oneround-trip time. When there is a read-write conflict, however, writersmay need to propose a MIN-TIMESTAMP in the appropriate voting round inorder to resolve the decision. When choosing the minimum timestamp topropose, the at least one DS processing unit 410 selects the largestrevision in order to guarantee that the revision set in the readtimestamp cache 458 is honored.

In embodiments, after the MIN-TIMESTAMP is decided, the at least one DSprocessing unit 410 resolves the WINNING-SUCCESSOR vote, with theconstraint that any proposals with a revision less than or equal to thechosen MIN-TIMESTAMP cannot win. In embodiments, if a writer's ownproposal is below this minimum timestamp and if there is no otherproposal to push forward, the write fails with a late write error andretries with a higher WINNING-SUCCESSOR revision.

In embodiments, when a reader at timestamp T successfully updates theread timestamp cache 458 on the DS storage units 430-1, 430-2, . . . ,430-n without a contest for the incumbent revision, the reader knowsthat some timestamp greater than or equal to T will win MIN-TIMESTAMPvote, so the reader can fallback without making any proposals that wouldopen that contest. On the other hand, in embodiments, if the readerfails to update the read timestamp cache 458, the reader drives theMIN-TIMESTAMP vote to completion by opening a new contest and resolvingthe MIN-TIMESTAMP decision. If the chosen MIN-TIMESTAMP is greater thanor equal to T for the current incumbent revision, the reader can safelyrestore the incumbent. Otherwise, the reader pushes the contest tocompletion before returning. Because a reader may encounter an opencontest with no restorable proposals, the reader may re-propose theexisting incumbent data at a new timestamp greater than or equal to T inorder to proceed.

FIG. 5 depicts a flowchart of an exemplary method for reading multiplesources (data objects) consistently in a DSN. The method of FIG. 5 isperformed by the DS processing unit program module 420 of the DSprocessing unit 410 and the DS storage unit program module 440 of the DSstorage units 430-1, 430-2, . . . , 430-n in accordance with aspects ofthe invention. The steps of the method are performed in the environmentof FIG. 4 and are described with reference to the elements shown in FIG.4.

At step 500, the DS processing unit 410 receives a read request for asource. In embodiments, the DS processing unit program module 420 of theDS processing unit 410 receives a request, from the client computersystem 460, to perform a consistent read of a source (data object) S,which is stored across a plurality of the DS storage units 430-1, 430-2,. . . , 430-n. In embodiments, the data storage 450 of each of the DSstorage units 430-1, 430-2, . . . , 430-n stores one or more slices, aplurality of which make up the source S. In embodiments, the requestreceived from the client computer system 460 optionally includes adesired read timestamp.

Still referring to FIG. 5, at step 510, the DS processing unit 410determines a timestamp. In embodiments, the DS processing unit programmodule 420 of the DS processing unit 410 determines a timestamp Tc forthe read request using the relative hybrid logical clock 427. Inembodiments, the relative hybrid logical clock 427 is a hybrid logicalclock that provides ordering of events based on the relative systemclock 425 and the logical counter 426. In other embodiments, in the casethat the read request received at step 500 includes the desired readtimestamp, the DS processing unit 410 determines the timestamp Tc basedon the desired read timestamp.

Still referring to FIG. 5, at step 520, the DS processing unit 410issues a read request for the source at the determined timestamp. Inembodiments, the DS processing unit program module 420 of the DSprocessing unit 410 issues the read request for the source S at thetimestamp Tc determined at step 510 to a plurality of the DS storageunits 430-1, 430-2, . . . , 430-n.

Still referring to FIG. 5, at step 530, the DS storage units 430-1,430-2, . . . , 430-n determine a current timestamp. In embodiments, theDS storage unit program module 440 of the DS storage units 430-1, 430-2,. . . , 430-n, in response to receiving the read request issued by theDS processing unit 410 at step 520, determines a current timestamp Tsusing the absolute clock 457. In embodiments, the absolute clock 457 isa clock that provides ordering of events based on the system clock 455and the logical counter 456. In embodiments, at step 530, the DS storageunit program module 440 of the DS storage units 430-1, 430-2, . . . ,430-n also determines clock skew based on the difference between Tc andTs. In embodiments, in the event that |Ts−Tc| is larger than apredetermined maximum acceptable skew, the DS storage unit programmodule 440 of the DS storage units 430-1, 430-2, . . . , 430-n rejectsthe read request issued by the DS processing unit 410 at step 520.

Still referring to FIG. 5, at step 540, the DS storage units 430-1,430-2, . . . , 430-n retrieve data from data storage 450. Inembodiments, the DS storage unit program module 440 of the DS storageunits 430-1, 430-2, . . . , 430-n, in response to the read requestissued by the DS processing unit program module 420 of the DS processingunit 410 at step 520, reads the source S at time Tc from the datastorage 450, which includes retrieving at least (1) all open contentswith incumbent revision<Tc and (2) the highest promoted revision<=Tc.

Still referring to FIG. 5, at step 550, the DS storage units 430-1,430-2, . . . , 430-n update the read timestamp cache 458. Inembodiments, the DS storage unit program module 440 of the DS storageunits 430-1, 430-2, . . . , 430-n update the read timestamp cache 458.In particular, in the read timestamp cache 458, the DS storage unitprogram module 440 sets a value at an array index or other locationcorresponding to the source S to the maximum of the value stored at thatindex and Tc. In other words, the DS storage unit program module 440sets cache[S]=max(cache[S], Tc) in the read timestamp cache 458.

Still referring to FIG. 5, at step 560, the DS storage units 430-1,430-2, . . . , 430-n respond to the DS processing unit 410 with the dataretrieved at step 540 and the timestamp Ts determined at step 530. Inembodiments, the DS storage unit program module 440 of the DS storageunits 430-1, 430-2, . . . , 430-n responds to the DS processing unit 410with the data retrieved at step 540 and the timestamp Ts determined atstep 530.

Still referring to FIG. 5, at step 570, the DS processing unit 410receives the responses from the DS storage units 430-1, 430-2, . . . ,430-n from step 560 and calculates a new clock offset. In embodiments,the DS processing unit program module 420 of the DS processing unit 410receives the responses from the DS storage unit program module 440 ofthe DS storage units 430-1, 430-2, . . . , 430-n from step 560,calculates a new estimated clock offset using the received timestamps Tsfrom step 560, and updates the relative hybrid logical clock 427 usingthe estimated clock offset.

Still referring to FIG. 5, at step 580, the DS processing unit 410returns a response to the read request from step 500. In embodiments,the DS processing unit program module 420 of the DS processing unit 410uses the responses received from the DS storage unit program module 440of the DS storage units 430-1, 430-2, . . . , 430-n at step 560 togenerate a response to the read request from step 500 and return theresponse to the client computer system 460.

FIG. 6 depicts a flowchart of an exemplary method for writing a revisionof a source (data object) in a DSN. The method of FIG. 6 is performed bythe DS processing unit program module 420 of the DS processing unit 410and the DS storage unit program module 440 of the DS storage units430-1, 430-2, . . . , 430-n in accordance with aspects of the invention.The steps of the method are performed in the environment of FIG. 4 andare described with reference to the elements shown in FIG. 4.

At step 600, the DS processing unit 410 receives a request to write arevision of a source. In embodiments, the DS processing unit programmodule 420 of the DS processing unit 410 receives a request, from theclient computer system 460, to write a revision of a source (dataobject) S. In embodiments, the revision of the source S is a different(e.g., newer) version of the source S (e.g., an updated version of adata object that was edited, revised, or otherwise changed by the clientcomputer system 460) that is to be divided into a plurality of slices bythe DS processing unit 410 and stored across the plurality of the DSstorage units 430-1, 430-2, . . . , 430-n in the data storage 450.

Still referring to FIG. 6, at step 605, the DS processing unit 410determines a current timestamp. In embodiments, the DS processing unitprogram module 420 of the DS processing unit 410 determines a currenttimestamp Tc for the write request using the relative hybrid logicalclock 427.

Still referring to FIG. 6, at step 610, the DS processing unit 410 sendsa proposal with an update revision for the source to the DS storageunits 430-1, 430-2, . . . , 430-n. In embodiments, the DS processingunit program module 420 of the DS processing unit 410 sends the proposalP with the update revision for the source S, including the currenttimestamp Tc determined at step 605 and a client ID corresponding to theclient computer system 460 from which the request was received at step600, to the DS storage unit program module 440 of the DS storage units430-1, 430-2, . . . , 430-n.

Still referring to FIG. 6, at step 615, the DS storage units 430-1,430-2, . . . , 430-n receive the proposal and persist the proposal inthe appropriate context. In embodiments, the DS storage unit programmodule 440 of the DS storage units 430-1, 430-2, . . . , 430-n receivesthe proposal sent by the DS processing unit program module 420 of the DSprocessing unit 410 at step 610 and persists the proposal in the datastorage 450.

Still referring to FIG. 6, at step 620, the DS storage units 430-1,430-2, . . . , 430-n determine a current timestamp based on a localabsolute clock. In embodiments, the DS storage unit program module 440of the DS storage units 430-1, 430-2, . . . , 430-n, in response toreceiving the proposal from the DS processing unit 410 at step 610,determines a current timestamp Ts using the absolute clock 457. Inembodiments, at step 620, the DS storage unit program module 440 of theDS storage units 430-1, 430-2, . . . , 430-n also determines clock skewbased on the difference between Tc and Ts. In embodiments, in the eventthat |Ts−Tc| is larger than a predetermined maximum acceptable skew, theDS storage unit program module 440 of the DS storage units 430-1, 430-2,. . . , 430-n rejects the proposal P received at step 615 and refuses tovote for the proposal P.

Still referring to FIG. 6, at step 625, the DS storage units 430-1,430-2, . . . , 430-n, when opening a new contest based on the proposal Preceived at step 615, set a vote for MIN-TIMESTAMP to a value stored inthe read timestamp cache 458. In embodiments, the DS storage unitprogram module 440 of the DS storage units 430-1, 430-2, . . . , 430-n,in response to the proposal P received at step 615 not being a subjectof an existing contest, opens a new contest based on the proposal P andsets a vote for MIN-TIMESTAMP to a value stored in the read timestampcache 458 at an array index or other location corresponding to thesource S.

Still referring to FIG. 6, at step 630, the DS storage units 430-1,430-2, . . . , 430-n respond to the DS processing unit 410 with acontest list, the timestamp determined at step 620, and theMIN-TIMESTAMP determined at step 625. In embodiments, the DS storageunit program module 440 of the DS storage units 430-1, 430-2, . . . ,430-n responds to the DS processing unit program module 420 of the DSprocessing unit 410 with the contest list, the timestamp Ts determinedat step 620, and the MIN-TIMESTAMP determined at step 625. Inembodiments, the MIN-TIMESTAMP is the lowest timestamp for which the DSstorage unit 430-1, 430-2, . . . , 430-n will vote.

Still referring to FIG. 6, at step 635, the DS processing unit 410receives the responses from the DS storage units 430-1, 430-2, . . . ,430-n and calculates a new estimated clock offset. In embodiments, theDS processing unit program module 420 of the DS processing unit 410receives the responses from the DS storage units 430-1, 430-2, . . . ,430-n sent at step 630, calculates a new estimated clock offset usingthe timestamps Ts in the responses, and updates the relative hybridlogical clock 427 using the new estimated clock offset.

Still referring to FIG. 6, at step 640, the DS processing unit 410resolves the MIN-TIMESTAMP decision. In embodiments, the DS processingunit program module 420 of the DS processing unit 410 resolves theMIN-TIMESTAMP decision based on the responses received from the DSstorage units 430-1, 430-2, . . . , 430-n at step 635, using a Paxosprotocol or other voting protocol. In embodiments, if the DS processingunit program module 420 of the DS processing unit 410 is unable toresolve the MIN-TIMESTAMP decision, the DS processing unit programmodule 420 proposes a new MIN-TIMESTAMP, requests new responses from theDS storage unit program module 440 of the DS storage units 430-1, 430-2,. . . , 430-n, and then makes another attempt to resolve theMIN-TIMESTAMP decision based on the responses received from the DSstorage units 430-1, 430-2, . . . , 430-n.

Still referring to FIG. 6, at step 645, the DS processing unit 410resolves the WINNING-SUCCESSOR decision, taking into account theMIN-TIMESTAMP determined at step 640. In embodiments, the DS processingunit program module 420 of the DS processing unit 410 resolves theWINNING-SUCCESSOR decision based on the responses received from the DSstorage units 430-1, 430-2, . . . , 430-n at step 635 and taking intoaccount the MIN-TIMESTAMP determined at step 640, using a Paxos protocolor other voting protocol.

Still referring to FIG. 6, at step 650, the DS processing unit 410 andthe DS storage units 430-1, 430-2, . . . , 430-n complete the writebased on the WINNING-SUCCESSOR determined at step 645. In embodiments,the DS processing unit program module 420 of the DS processing unit 410causes the DS storage unit program module 440 of the DS storage units430-1, 430-2, . . . , 430-n to complete the write of the source S to thedata storage 450 based on the WINNING-SUCCESSOR determined at step 645.

In embodiments, a system and method are provided for providingconsistent reads in a CASN distributed transaction protocol, the methodincluding the steps of: issuing, via a DS processing unit 410, multipleread requests with the same client timestamp (Tc); for each readrequest, issuing, via a grid layer, requests with the timestamp (Tc),and ensuring that either the highest revision less than or equal to (Tc)or an appropriate error is returned; and when a quorum of DS storageunits 430-1, 430-1, . . . , 430-n cannot service a read at a particulartimestamp because old data has already been discarded, aborting thetransaction at the DS processing unit 410, and retrying the entiretransaction.

In embodiments, a service provider could offer to perform the processesdescribed herein. In this case, the service provider can create,maintain, deploy, support, etc., the computer infrastructure thatperforms the process steps of the invention for one or more customers.These customers may be, for example, any business that uses cloudcomputing technology. In return, the service provider can receivepayment from the customer(s) under a subscription and/or fee agreementand/or the service provider can receive payment from the sale ofadvertising content to one or more third parties.

In still additional embodiments, the invention provides acomputer-implemented method, via a network. In this case, a computerinfrastructure, such as computer system/server 12 (FIG. 1), can beprovided and one or more systems for performing the processes of theinvention can be obtained (e.g., created, purchased, used, modified,etc.) and deployed to the computer infrastructure. To this extent, thedeployment of a system can comprise one or more of: (1) installingprogram code on a computing device, such as computer system/server 12(as shown in FIG. 1), from a computer-readable medium; (2) adding one ormore computing devices to the computer infrastructure; and (3)incorporating and/or modifying one or more existing systems of thecomputer infrastructure to enable the computer infrastructure to performthe processes of the invention.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A method comprising: receiving, by a dispersedstorage (DS) processing unit in a dispersed storage network, a requestto write a revision of a data object in the dispersed storage network,wherein the data object is stored as slices at a plurality of DS storageunits in the dispersed storage network and the revision of the dataobject is a newer version of the data object; sending, by the DSprocessing unit, a proposal with the revision of the data object to theplurality of DS storage units; receiving, by the DS processing unit, aresponse to the proposal from each of the plurality of DS storage units,the response including a proposed minimum timestamp corresponding to thedata object; determining, by the DS processing unit, a minimum timestampfor the data object based on the proposed minimum timestamps receivedfrom the plurality of DS storage units; determining, by the DSprocessing unit, a version of the data object written in the dispersedstorage network based on the minimum timestamp; and completing the writeof the revision of the data object in the dispersed storage networkbased on the determined version.
 2. The method according to claim 1,further comprising determining, by the DS processing unit, a currenttimestamp of the DS processing unit, and wherein the proposal includesthe current timestamp of the DS processing unit.
 3. The method accordingto claim 2, wherein the response to the proposal received from each ofthe plurality of DS storage units further comprises a current timestampof the DS storage unit.
 4. The method according to claim 3, furthercomprising the DS processing unit using a Paxos protocol to determinethe minimum timestamp based on the responses to the proposal receivedfrom the DS storage units.
 5. The method according to claim 1, whereinthe determining the version of the data object written in the dispersedstorage network comprises determining a WINNING-SUCCESSOR using a Paxosprotocol based on the responses to the proposal received from the DSstorage units.
 6. The method according to claim 5, wherein thecompleting the write of the revision of the data object is performedbased on the WINNING-SUCCESSOR.
 7. The method according to claim 1,wherein: the DS processing unit comprises a relative system clock, alogical counter, and a relative hybrid logical clock; the relativehybrid logical clock is a hybrid logical clock that provides ordering ofevents based on the relative system clock and the logical counter; theDS processing unit determines a current timestamp for the write requestusing the relative hybrid logical clock; and the DS processing usingsends the current timestamp with the proposal.
 8. A computer programproduct comprising: one or more computer readable storage media, andprogram instructions collectively stored on the one or more computerreadable storage media, the program instructions comprising: programinstructions to receive, by the DS processing unit in a dispersedstorage network, a read request for a data object, wherein the dataobject is stored as slices at a plurality of DS storage units in thedispersed storage network, the read request includes a requesttimestamp, and the request is received from a client computer system;program instructions to determine, by the DS processing unit, a currenttimestamp; program instructions to retrieve, by the DS processing unit,data from storage corresponding to the data object at the requesttimestamp; program instructions to update, by the DS processing unit, aread timestamp cache based on the current timestamp; programinstructions to provide, by the DS processing unit, the retrieved datato a dispersed storage (DS) processing unit; and program instructions togenerate, by the DS processing unit, a response to the read request andreturn the response to the client computer system.
 9. The computerprogram product according to claim 8, wherein the updating the readtimestamp cache comprises setting a value at an index corresponding tothe data object to a maximum of a value presently stored at the indexand the current timestamp.
 10. The computer program product according toclaim 8, further comprising program instructions to determine adifference between the request timestamp and the read timestamp.
 11. Thecomputer program product according to claim 10, further comprisingprogram instructions to, in response to the difference between therequest timestamp and the read timestamp exceeding a predeterminedmaximum acceptable skew, reject the read request.
 12. The computerprogram product according to claim 8, wherein request timestamp is ahybrid logical clock timestamp.
 13. The computer program productaccording to claim 8, wherein: the DS processing unit comprises arelative system clock, a logical counter, and a relative hybrid logicalclock; the relative hybrid logical clock is a hybrid logical clock thatprovides ordering of events based on the relative system clock and thelogical counter; and the DS processing unit determines the currenttimestamp using the relative hybrid logical clock.
 14. A systemcomprising: a hardware processor, a computer readable memory, and one ormore computer readable storage media associated with a computing device,wherein the computing device is a dispersed storage (DS) processingunit; program instructions to receive, by the DS processing unit in adispersed storage network, a request to write a revision of a dataobject in the dispersed storage network, wherein the data object isstored as slices at a plurality of DS storage units in the dispersedstorage network and the revision of the data object is a newer versionof the data object; program instructions to send, by the DS processingunit, a proposal with the revision of the data object to the pluralityof DS storage units; program instructions to receive, by the DSprocessing unit, a response to the proposal from each of the pluralityof DS storage units, the response including a proposed minimum timestampcorresponding to the data object; program instructions to determine, bythe DS processing unit, a minimum timestamp for the data object based onthe proposed minimum timestamps received from the plurality of DSstorage units; program instructions to determine, by the DS processingunit, a version of the data object written in the dispersed storagenetwork based on the minimum timestamp; and program instructions tocompleting a write of the revision of the data object in the dispersedstorage network based on the determined version, wherein the programinstructions are collectively stored on the one or more computerreadable storage media for execution by the hardware processor via thecomputer readable memory.
 15. The system according to claim 14, furthercomprising program instructions to determine a current timestamp of theDS processing unit, wherein the proposal includes the current timestampof the DS processing unit.
 16. The system according to claim 15, whereinthe response to the proposal received from each of the plurality of DSstorage units further comprises a current timestamp of the DS storageunit.
 17. The system according to claim 16, further comprising programinstructions to use a Paxos protocol to determine the minimum timestampbased on the responses to the proposal received from the DS storageunits.
 18. The system according to claim 14, wherein the determining theversion of the data object written in the dispersed storage networkcomprises determining a WINNING-SUCCESSOR using a Paxos protocol basedon the responses to the proposal received from the DS storage units. 19.The system according to claim 18, wherein the plurality of completingthe write of the revision of the data object is performed based on theWINNING-SUCCESSOR.
 20. The system according to claim 14, wherein: the DSprocessing unit comprises a relative system clock, a logical counter,and a relative hybrid logical clock; the relative hybrid logical clockis a hybrid logical clock that provides ordering of events based on therelative system clock and the logical counter; the DS processing unitdetermines a current timestamp for the write request using the relativehybrid logical clock; and the DS processing using sends the currenttimestamp with the proposal.